1. Field of the Invention
The present invention relates to a zoom lens system which is thin in a direction of depth and an image pickup apparatus such as a video camera, a digital camera or the like which has the zoom lens system.
2. Description of the Prior Art
Digital camera (electronic cameras) have recently been attracting attentions as cameras of a next generation which are to substitute for a silver salt 35 mm film (the so-called Leica size) camera.
These digital cameras are known as cameras ranging from high performance cameras for business to portable prevailing type.
For realizing a portable prevailing type video camera forming an object of the present invention, a video camera or a digital camera which provides a high quality image and is thin in a direction of depth in particular, a key lies in shortening a distance as measured from a most object side surface to an image pickup surface of an optical system, a zoom optical system in particular.
These have recently been known image pickup apparatuses (cameras) which have collapsible mount type lens barrels which allow optical systems to be pushed out of camera bodies at a photographing time and to be accommodated into the camera bodies at a carrying time.
However, thicknesses of these cameras having the collapsible mount type lens barrels in conditions where optical systems are collapsed are largely different dependently on types of the optical systems and filters used.
The so-called positive lens unit leading type zoom lens system in which a lens unit disposed on a most object side in an optical system has positive refractive power as in a lens system disclosed by Japanese Laid-Open Patent Application No. Hei 11-258507 for setting high specifications for a zoom ratio, an F number and the like in particular uses thick lens elements, has large dead spaces and cannot have a thickness so small even in a collapsed condition.
In contrast, a negative lens unit leading type zoom lens system in which a negative lens unit is disposed on a most object side and which consists of two or three lens unit are disposed is advantageous for adopting a collapsible mount type lens barrel.
Furthermore, a zoom lens system disclosed by Japanese Laid-Open Patent Application No. Hei 11-52246 cannot thin a thickness of a camera even in a collapsed condition since the zoom lens system uses lens units each of which comprises a large number of lens components and in which a positive lens component is disposed on a most object side.
Out of currently known zoom lens systems, those which are disclosed by Japanese Laid-Open Patent Application No. Hei 11-194274. No. Hei 11-287953 and No-2000-1997 are suited for use with cameras using electronic image pickup devices, have favorable imaging performance such as zoom ratios, F numbers and the like, and provide smallest collapsed thickness.
Though it is proper to shallow a location of an entrance pupil for thinning a first lens unit in each of these conventional examples, a lens unit is obliged to have an enhanced magnification. When the second lens unit has the enhanced magnification and a large share of refractive power of a lens system as a whole, however, it is impossible to thin the second lens unit itself, difficult to correct aberrations and undesirably enlarge influences due to manufacturing errors.
Furthermore, it is proper to configure an image pickup device compact for configuring a camera thin and compact, but it is necessary for compact configuration of the image pickup device without changing a number of picture elements to arrange the picture elements at a narrower pitch and make up for an insufficient sensitivity with an optical system. Furthermore, the compact configuration of the is not preferable since such a configuration allows diffraction to produce an influence.
An object of the present invention is to provide a zoom lens system comprising, in order from the object side, a first lens unit having negative refractive power, a second lens unit having positive refractive power and a third lens unit having positive refractive power, wherein the second lens unit is moved only on the object side along an optical axis to change a magnification from a wide-angle position to a telephoto position, wherein the second lens unit consists of four lens elements including, in order from the object side, two positive lens elements and a negative lens element, and satisfies the following conditions (1) and (2).
1.3 less than |xcex223T| less than 2.0 xe2x80x83xe2x80x83(1) 
0.25 less than |f2/f3| less than 1.5 xe2x80x83xe2x80x83(2) 
An object of the present invention is to provide a zoom lens system having the above described composition, wherein the second lens unit consists of four lens elements including, in order from the object side, a positive lens element, and a cemented lens component consisting of a positive lens element and a negative lens element.
Another object of the present invention is to provide a zoom lens system comprising, in order from the object side, a first lens unit having negative refractive power, a second lens unit having positive refractive power and a third lens unit having positive refractive power, wherein the above described third lens unit is moved so as to prolong a distance from the above described third lens unit to an image surface to change a magnification from a wide-angle position to a telephoto position, and wherein the above described third lens unit consists only of a positive lens component, and satisfies the following conditions (11), (12), (13) and (14):
|fw/f2R| less than 0.1 xe2x80x83xe2x80x83(11) 
0.89 less than f3/fT less than 2.8 xe2x80x83xe2x80x83(12) 
1.2 less than |xcex223T| less than 2.0 xe2x80x83xe2x80x83(13) 
1/xcex22T less than 0.25 xe2x80x83xe2x80x83(14) 
Another object of the present invention is to provide an image pickup apparatus comprising a zoom lens system and an image pickup device. The zoom lens system comprises, in order from the object side, a first lens unit having negative refractive power, a second lens unit having positive refractive power and a third lens unit having positive refractive power, wherein the second lens unit is moved only on the object side to change a magnification from a wide-angle position to a telephoto position, and wherein the above described the second lens unit consists of four lens element including, in order from the object side, two positive lens elements and a negative lens element and satisfies the following conditions (1) and (2):
1.3 less than |xcex223T| less than 2.0 xe2x80x83xe2x80x83(1) 
0.25 less than |f2/f3| less than 1.5 xe2x80x83xe2x80x83(2) 
Another object of the present invention is to provide an image pickup apparatus comprising a zoom lens system and an image pickup device, wherein the zoom lens system comprises, in order from the object side, a first lens unit having negative refractive power, a second lens unit having positive refractive power and a third lens unit having positive refractive power, wherein the above described third lens unit is moved so as to prolong a distance from the above described third lens unit to am image surface to change a magnification from a wide-angle position to a telephoto position, and wherein the above described third lens unit consists of a positive lens component, and satisfies the following conditions (11), (12), (13) and (14):
|fw/f2R| less than 0.1 xe2x80x83xe2x80x83(11) 
0.89 less than f3/fT less than 2.8 xe2x80x83xe2x80x83(12) 
1.1 less than |xcex223T| less than 2 xe2x80x83xe2x80x83(13) 
1/xcex22T less than 0.25 xe2x80x83xe2x80x83(14) 